Track step nesting in StreamEngine.

recipe_modules/generator_script/example.py

Index: recipe_modules/generator_script/example.py
diff --git a/recipe_modules/generator_script/example.py b/recipe_modules/generator_script/example.py
index a57e0d79412280d3c9515e9e99879da8b0b9e648..d82657846cd13b94966a3fe5967a695a265ad2f0 100644
--- a/recipe_modules/generator_script/example.py
+++ b/recipe_modules/generator_script/example.py
@@ -73,6 +73,20 @@ def GenTests(api):
   )
 
   yield (
+    api.test('nested') +
+    api.properties(script_name='nested.py') +
+    api.generator_script(
+      'nested.py',
+      {'name': 'grandparent', 'cmd': ['echo', 'grandparent']},
+      {'name': 'parent', 'step_nest_level': 1, 'cmd': ['echo', 'parent']},
+      {'name': 'child', 'step_nest_level': 2, 'cmd': ['echo', 'child']},
+      {'name': 'sibling', 'step_nest_level': 2, 'cmd': ['echo', 'sibling']},
+      {'name': 'uncle', 'step_nest_level': 1, 'cmd': ['echo', 'uncle']},
+      {'name': 'cousin', 'step_nest_level': 2, 'cmd': ['echo', 'cousin']},
+    )
+  )
+
+  yield (
     api.test('malformed_command') +
     api.properties(script_name='malformed.py') +
     api.generator_script(